CN115244201B - Hot-rolled steel sheet and method for producing same - Google Patents

Abstract

The hot-rolled steel sheet has a predetermined chemical composition and a metallographic structure, and has a ratio of a maximum depth in a region where a rotation angle of a (011) pole near the normal line of one surface to a normal line of the one surface is 5 DEG or less to a maximum depth in a region where a rotation angle of a (011) pole near the normal line of the other surface to a normal line of the other surface is 5 DEG or less, of 1.00 to 1.20, and a tensile strength of 1150MPa or more.

Description

Hot-rolled steel sheet and manufacturing method thereof

technical field

The present disclosure relates to a hot-rolled steel sheet and a manufacturing method thereof.

This application claims priority based on Japanese Patent Application No. 2020-082655 for which it applied in Japan on May 8, 2020, and uses the content here.

Background technique

In recent years, the weight reduction of automobiles and various mechanical parts has been promoted. Rigidity can be ensured by designing parts in optimal shapes, thereby reducing the weight of automobiles and various mechanical parts. Furthermore, in blank-formed parts such as press-formed parts, weight reduction can be achieved by reducing the plate thickness of the part material. However, high-strength materials need to be used when it is desired to reduce the plate thickness while ensuring the strength characteristics of parts such as static breaking strength and yield strength. In particular, studies have begun on the application of steel plates exceeding 780 MPa to automotive running parts such as lower arms, drawbars, and steering knuckles. These automobile undercarriage parts are manufactured by subjecting steel sheets to flanging, drawing flanges, and bending. Therefore, steel sheets used for these automobile underbody parts are required to have excellent formability.

For example, Patent Document 1 discloses a hot-rolled steel sheet in which the grain size and length of prior-austenite are controlled by setting the finishing temperature and rolling reduction within predetermined ranges in the hot-rolling process. Aspect ratio, reducing anisotropy.

Patent Document 2 discloses a cold-rolled steel sheet having improved toughness by setting the rolling ratio and the average strain rate within appropriate ranges within a predetermined finish rolling temperature range in a hot rolling step.

In order to further reduce the weight of automobiles and various mechanical parts, there are also plans to apply cold-rolled steel sheets to thick steel sheets for automobile running parts. The techniques described in Patent Document 1 and Patent Document 2 are effective in the manufacture of automobile running parts to which high-strength steel sheets are applied. In particular, these techniques are important insights for obtaining effects related to the formability and impact properties of undercarriage parts of automobiles having complex shapes.

However, vehicle running parts are always subjected to vibrations due to their own weight, rotation, repeated loads due to riding, and the like. Therefore, durability as a component is an important characteristic. As mentioned above, running parts of automobiles are subjected to various moldings. In the planar portion near the inner side of the R portion subjected to bending or bending recovery molding, there are many portions where contact with the die is weak. In the flat portion near the inner side of such R portion, due to the development of unevenness of the surface layer due to molding and the contact of the mold under a weak load, it becomes a surface texture in which relatively sharp concave portions are periodically formed (hereinafter, such Changes in surface properties are recorded as molding damage).

For example, in Non-Patent Document 1, the development of unevenness in the surface layer due to such forming near the inside of the bend is simulated by uniaxial deformation, and the fatigue characteristics of the steel sheet in contact with the mold are investigated. It was found that the fatigue properties of these steel sheets were lowered by the recesses, but the change was different depending on the metallographic structure. In steel sheets over 780 MPa class used in automobile chassis parts, the volume ratio of hard structures is increased in order to express strength, but there is no technology to sufficiently improve the fatigue characteristics of steel sheets subjected to forming damage in such a strength region.

prior art literature

patent documents

Patent Document 1: Japanese Patent No. 5068688

Patent Document 2: Japanese Patent No. 3858146

non-patent literature

Non-Patent Document 1: Plasticity and Processing (Journal of the Japan Society for Plastic Processing) Vol. 57 No. 666 (2016-7) p660-p666

Contents of the invention

The technical problem to be solved by the invention

The inventors of the present invention have conducted technological developments to reduce molding damage and improve fatigue properties. The inventors of the present invention have newly found that when the depth of the recess exceeds a certain constant value, the fatigue properties of the hot-rolled steel sheet deteriorate significantly.

The higher the strength of the plate, the higher the notch sensitivity. Therefore, since a high-strength steel sheet exceeding 780 MPa class is applied to automobile running parts, it is necessary to improve the fatigue characteristics of the formed damaged portion. In order to reduce the depth of the concave part, one way is to increase the contact surface pressure of the mold. However, the contact surface pressure of the mold is a molding factor that controls the amount of plastic flow during molding. In addition, when pressing a high-strength steel sheet into a complicated shape, it is difficult to increase the surface pressure with a predetermined pressing load.

In view of the above circumstances, an object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent formability, and excellent fatigue properties at formed damaged parts, and a method for producing the same.

Technical means used to solve technical problems

The inventors of the present invention conducted innovative research and found that the depth of the concave portion focusing on the forming damage portion is derived from the uneven deformation of the front and back surfaces of the hot-rolled steel sheet during forming, and the depth of the concave portion after the mold contact (after forming) is related to the steel plate. There is a relationship between the characteristics of the macroscopic crystal orientation distribution on the surface and the back. The inventors of the present invention have found that by setting appropriate chemical components and metallographic structures for obtaining high strength and excellent formability, and further controlling specific crystal orientations in the thickness direction of the front and back surfaces, it is possible to reduce the risk of forming damage. The depth of the concave portion can thereby suppress the reduction of the fatigue properties of the molding damaged portion.

The gist of the present invention completed based on the above findings is as follows.

(1) In the hot-rolled steel sheet according to one aspect of the present invention, the chemical composition contains C: 0.085 to 0.190%,

Si: 0.40～1.40%,

Mn: 1.70～2.75%,

Al: 0.01 to 0.55%,

Nb: 0.006 to 0.050%,

P: less than 0.080%,

S: 0.010% or less,

N: 0.0050% or less,

Ti: 0.004～0.180%,

B: 0.0004～0.0030%,

Mo: 0～0.150%,

V: 0～0.300%,

Cr: 0 to 0.500%, and

Ca: 0～0.0020%,

The remainder is Fe and impurities;

In the metallographic structure of the 1/4 position from the surface in the thickness direction and the 1/2 position from the surface in the thickness direction, in volume %

Retained austenite is 3.0-12.0%,

Bainite is more than 75.0% and less than 97.0%,

Ferrite is less than 10.0%,

Martensite is less than 10.0%,

Pearlite is less than 3.0%;

In the metallographic structure of the area from the surface to a position 100 μm away from the surface in the plate thickness direction, the average grain size of prior austenite grains is 25.0 μm or less;

The maximum depth of the region where the rotation angle between the normal of the one of the faces and the (011) pole near the normal of the one of the faces is 5° or less, and the maximum depth in the other face The ratio of the normal of the other said surface to the maximum depth of the region where the rotation angle of the (011) pole near the said normal of said another said surface is 5° or less is 1.00-1.20;

The tensile strength is above 1150MPa.

(2) In the hot-rolled steel sheet described in (1) above, the chemical composition may contain, in mass %, one or more of the following elements:

Mo: 0.030～0.150%,

V: 0.050～0.300%,

Cr: 0.050 to 0.500%, and

Ca: 0.0006 to 0.0020%.

(3) The method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention is the method for manufacturing a hot-rolled steel sheet described in (1) or (2) above, comprising:

In the continuous casting process, when the slab having the chemical composition described in the above (1) is continuously cast, the average cooling rate gradient from the meniscus to the surface temperature in the region of 1.0 m from the meniscus is Continuous casting at 0.20-15.00°C/ ^s2 to obtain the slab;

heating step, heating the slab to above 1200°C;

A hot rolling step, after rough rolling the heated slab, rolling in the temperature range of 870 to 980°C at a total reduction rate of 80% or more in the temperature range of 870°C to 980°C The finish rolling is carried out in such a way that the elapsed time between the rolling stands is 4.00 seconds or less;

cooling process, cooling to a temperature range of 300-550°C; and

In the coiling step, coiling is performed so that the coiling temperature becomes the above-mentioned temperature range of 300 to 550°C.

(4) The method for producing a hot-rolled steel sheet as described in (3) above may include a tempering step of maintaining the steel sheet at a temperature ranging from 200°C to less than 450°C after the coiling step. 90 to 80000 seconds.

(5) The method for producing a hot-rolled steel sheet as described in (3) or (4) above may include a plating step for the hot-rolled steel sheet after the coiling step or the tempering step The final hot-rolled steel sheet was subjected to a hot-dip galvanizing treatment with a heat history in a temperature range of 450 to 495° C. and a residence time of 75 seconds or less.

Invention effect

According to the above aspect of the present invention, there can be provided a hot-rolled steel sheet having high strength, excellent formability, and excellent fatigue properties at a formed damaged portion, and a method for producing the same. According to the above aspects of the present invention, since the molded damaged portion has excellent fatigue characteristics, the depth of the recess in the flat portion near the R portion formed when the R portion is formed can be reduced.

Description of drawings

FIG. 1 is a graph showing the relationship between the fatigue strength and the maximum depth ratio of a molded damaged portion in an example.

FIG. 2 is a graph showing the relationship between the fatigue strength of a forming damaged portion and the average grain size of prior-austenite grains in Examples.

3 is a graph showing the relationship between the average cooling rate gradient and the ratio of the maximum depth of the surface temperature from the meniscus to the region 1.0 m away from the meniscus in Examples.

Detailed ways

Hereinafter, the hot-rolled steel sheet of this embodiment will be described in detail. However, the present invention is not limited only to the configuration disclosed in this embodiment, and various changes can be made without departing from the gist of the present invention.

In addition, in the numerical limitation range described below with "-" interposed, the lower limit and the upper limit are included in this range. In numerical values expressed as "less than" or "more than", the value is not included in the numerical range. "%" with respect to chemical components all means "mass %".

The hot-rolled steel sheet of the present embodiment contains, in mass %: C: 0.085-0.190%, Si: 0.40-1.40%, Mn: 1.70-2.75%, Al: 0.01-0.55%, Nb: 0.006-0.050%, P: 0.080% or less, S: 0.010% or less, N: 0.0050% or less, Ti: 0.004 to 0.180%, B: 0.0004 to 0.0030%, Mo: 0 to 0.150%, V: 0 to 0.300%, Cr: 0 to 0.500% , Ca: 0 to 0.0020%, and the remainder: Fe and impurities. Each element will be described in detail below.

C: 0.085～0.190%

In addition to determining the strength of the hot-rolled steel sheet, C is one of the elements that affect the amount of retained austenite. If the C content is less than 0.085%, the volume fraction of retained austenite cannot be made to be 3.0% or more. Therefore, the C content is 0.085% or more. Preferably it is 0.115% or more.

On the other hand, when the C content exceeds 0.190%, the volume fraction of retained austenite increases, and the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the C content is 0.190% or less. Preferably it is 0.170% or less.

Si: 0.40～1.40%

Si is an element that increases the strength of a hot-rolled steel sheet through solid solution strengthening. In addition, Si is also an element that suppresses the formation of carbides such as pearlite. In order to obtain these effects, the Si content is 0.40% or more. Preferably it is 0.90% or more. When the Si content is less than 0.40%, the volume fraction of retained austenite is less than 3.0%, and the volume fraction of pearlite exceeds 3.0%.

On the other hand, if the Si content increases, the volume fraction of retained austenite increases, but if it exceeds 1.40%, the volume fraction of retained austenite exceeds 12.0%, and the hole expandability of the hot-rolled steel sheet decreases. In addition, since Si has a high ability to form oxides, if the Si content is excessive, oxides will be formed at welded parts or the chemical conversion treatability of the hot-rolled steel sheet will be deteriorated in the component manufacturing process. Therefore, the Si content is 1.40% or less. Preferably it is 1.30% or less.

Mn: 1.70～2.75%

Mn is an element required to increase the strength of a hot-rolled steel sheet. If the Mn content is less than 1.70%, the volume ratio of ferrite exceeds 10.0%, and a tensile strength of 1150 MPa or more cannot be obtained. Therefore, the Mn content is 1.70% or more. Preferably it is 1.80% or more.

On the other hand, if the Mn content exceeds 2.75%, the toughness of the cast slab deteriorates and hot rolling cannot be performed. Therefore, the Mn content is 2.75% or less. Preferably it is 2.70% or less.

Al: 0.01-0.55%

Al functions as a deoxidizer and is an element that improves the purity of steel. In order to obtain this effect, the Al content is 0.01% or more. Preferably it is 0.10% or more.

On the other hand, when the Al content exceeds 0.55%, casting becomes difficult. Therefore, the Al content is 0.55% or less. Al is an oxidizing element, and the Al content is preferably 0.30% or less in order to obtain the effect of further improving the continuous castability and the cost reduction effect.

Nb: 0.006～0.050%

Nb suppresses the abnormal grain growth of austenite grains in the hot rolling process, thereby reducing the depth of the concave portion of the forming damage. In order to obtain this effect, the Nb content is 0.006% or more. When the Nb content is 0.025% or more, the above-mentioned effects are saturated.

On the other hand, if the Nb content exceeds 0.050%, the toughness of the cast slab deteriorates and hot rolling cannot be performed. Therefore, the Nb content is 0.050% or less. Preferably it is 0.025% or less.

P: 0.080% or less

P is an impurity element inevitably mixed in the production process of the hot-rolled steel sheet. The higher the P content, the more embrittled the hot-rolled steel sheet. In the case of applying the hot-rolled steel sheet to automobile running parts, the P content can be allowed up to 0.080%. Therefore, the P content is 0.080% or less. Preferably it is 0.010% or less. In addition, if the P content is reduced to less than 0.0005%, the cost of removing P will increase significantly, so the P content can also be 0.0005% or more.

S: 0.010% or less

When molten steel contains a large amount of S, MnS is formed to degrade the ductility and toughness of the hot-rolled steel sheet. Therefore, the S content is 0.010% or less. Preferably it is 0.008% or less. In addition, if the S content is reduced to less than 0.0001%, the desulfurization cost will increase significantly, so the S content may be 0.0001% or more.

N: 0.0050% or less

N is an impurity element inevitably mixed in the production process of the hot-rolled steel sheet. When the N content exceeds 0.0050%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the N content is 0.0050% or less. Preferably it is 0.0040% or less. In addition, if the N content is reduced to less than 0.0001%, the steelmaking cost will increase significantly, so the N content may be 0.0001% or more.

Ti: 0.004～0.180%

Ti has the effect of enhancing the effect of containing B, which will be described later, by forming nitrides. In order to obtain this effect, the Ti content is 0.004% or more. Preferably it is 0.006% or more. In order to increase the effect of containing B by containing Ti, the Ti content may be 0.013% or more.

On the other hand, Ti is an element that deteriorates the toughness of the slab. When the Ti content exceeds 0.180%, slab cracks may occur frequently and the cost of solutionization may increase. Therefore, Ti is 0.180% or less. Preferably it is 0.140% or less, 0.100% or less.

B: 0.0004～0.0030%

B is an element that suppresses the formation of ferrite in the cooling step. In order to obtain this effect, the B content is 0.0004% or more. Preferably it is 0.0011% or more.

On the other hand, since the above effects are saturated even if B is contained in excess of 0.0030%, the B content is 0.0030% or less. Preferably it is 0.0020% or less.

The remainder of the chemical composition of the hot-rolled steel sheet of this embodiment may be Fe and impurities. In the present embodiment, the impurities are substances that are allowed to enter from ores as raw materials, waste materials, manufacturing environments, etc., within a range that does not adversely affect the hot-rolled steel sheet of the present embodiment.

The hot-rolled steel sheet of this embodiment may contain one or two or more of the group consisting of Mo, V, Cr, and Ca as optional elements instead of a part of Fe. The lower limit of the content when the above-mentioned arbitrary elements are not contained is 0%. Hereinafter, each arbitrary element will be described.

Mo: 0～0.150%

Mo is an element that improves the hardenability of steel, and may be contained as an element that adjusts the strength of the hot-rolled steel sheet. In order to securely obtain the above effects, the Mo content is preferably 0.030% or more. On the other hand, even if more than 0.150% of Mo is contained, the above-mentioned effects are saturated. Therefore, the Mo content is preferably 0.150% or less.

V: 0～0.300%

V has the effect of increasing the strength by forming fine carbides. In order to reliably obtain this effect, the V content is preferably 0.050% or more. However, if V is contained excessively, nitrides are formed in the steel, thereby degrading the toughness of the slab and making it difficult to pass through the slab. Therefore, the V content is preferably 0.300% or less.

Cr: 0～0.500%

Cr is an element that exhibits an effect similar to that of Mn. In order to reliably obtain the effect of improving the strength of the hot-rolled steel sheet, the Cr content is preferably 0.050% or more. On the other hand, even if more than 0.500% of Cr is contained, the above-mentioned effects are saturated. Therefore, the Cr content is preferably 0.500% or less.

Ca: 0～0.0020%

Ca improves local ductility and improves hole expandability by forming fine CaS. However, if the Ca content exceeds 0.0020%, the manufacturability will deteriorate due to the formation of oxides in the nozzle during continuous casting, and the formability will deteriorate due to the inclusion of these oxides. Therefore, the Ca content is preferably 0.0020% or less. In addition, in order to obtain the above effects, the Ca content is preferably 0.0006% or more.

The above-mentioned chemical composition of the hot-rolled steel sheet can be analyzed using a spark discharge emission spectroscopic analyzer or the like. In addition, for C and S, the values identified by measuring with an infrared absorption method by burning in an oxygen flow using a gas component analyzer or the like are used. In addition, for N, a value obtained by melting a test piece collected from a hot-rolled steel sheet in a helium stream and measuring it by a thermal conductivity method was used.

Next, the metallographic structure of the hot-rolled steel sheet of the present embodiment will be described. In addition to the effect of improving the strength and formability of the hot-rolled steel sheet, the characteristics of the metallographic structure are limited to the range that can improve the fatigue characteristics of the forming damage part.

In the metallographic structure of the hot-rolled steel sheet of the present embodiment at a position 1/4 from the surface in the thickness direction and a position 1/2 from the surface in the thickness direction, the retained austenite is 3.0% by volume. ～12.0%, 75.0% or more and less than 97.0% of bainite, less than 10.0% of ferrite, less than 10.0% of martensite, less than 3.0% of pearlite, from the surface to the plate from the surface In the metallographic structure of the region at the position of 100 μm in the thickness direction, the average grain size of the prior-austenite grains is 25.0 μm or less, and the normal line of the one of the surfaces is different from the normal line of the one of the surfaces. The maximum depth of the region where the rotation angle of the (011) pole near the normal line is 5° or less, and the normal line of the other said surface in the other surface is different from the normal line of the other said surface The ratio of the maximum depth in the region where the rotation angle of the (011) pole near the line is 5° or less is 1.00 to 1.20.

Each protocol will be described below.

Retained austenite: 3.0～12.0%

In order to improve the ductility of the hot-rolled steel sheet, the retained austenite needs to be 3.0% or more in volume ratio. In order to improve the fatigue properties of the hot-rolled steel sheet, the volume fraction of retained austenite is preferably 6.0% or more.

On the other hand, when the retained austenite volume ratio exceeds 12.0%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the volume fraction of retained austenite is 12.0% or less. Preferably it is 10.0% or less or 9.0% or less.

Bainite: more than 75.0% and less than 97.0%

Bainite is a structure that has predetermined strength and is excellent in balance between ductility and hole expandability. First, in order to make the total elongation 13.0% or more, it is necessary to make the volume ratio of retained austenite 3.0% or more and make the volume ratio of bainite less than 97.0%. Therefore, the volume fraction of bainite is less than 97.0%. Preferably it is 95.0% or less.

On the other hand, when the volume fraction of bainite is less than 75.0%, the homogeneity of the structure is lost, and the hole expandability deteriorates. Therefore, the volume ratio of bainite is 75.0% or more. Preferably it is 80.0% or more.

Ferrite: less than 10.0%

Ferrite has high deformability and is an effective structure for improving the ductility of the hot-rolled steel sheet, but if the volume ratio is too large, the strength of the hot-rolled steel sheet decreases. If the volume ratio of ferrite exceeds 10.0%, the strength of the hot-rolled steel sheet decreases, and the tensile strength becomes less than 1150 MPa. Therefore, the volume ratio of ferrite is 10.0% or less. Preferably it is 6.0% or less. The lower limit of the volume fraction of ferrite is not particularly limited, and may be 0%.

Martensite: less than 10.0%

Martensite has the effect of increasing the strength, but the local deformability is low, and the local elongation and hole expandability of the hot-rolled steel sheet decrease due to the increase in the volume ratio. If the volume ratio of martensite exceeds 10.0%, the hole expansion ratio of the hot-rolled steel sheet will be less than 35.0%. Therefore, the volume ratio of martensite is 10.0% or less. Preferably it is 6.0% or less. The lower limit of the volume fraction of martensite is not particularly limited, and may be 0%.

Pearlite: less than 3.0%

Pearlite is a structure that deteriorates the hole expandability of a hot-rolled steel sheet. If the volume ratio of pearlite exceeds 3.0%, the hole expansion ratio of the hot-rolled steel sheet will be less than 35.0%. Therefore, the volume ratio of pearlite is 3.0% or less. Preferably it is 1.5% or less. The lower limit of the volume fraction of pearlite is not particularly limited, and may be 0%.

Determination method of volume fraction of retained austenite

The volume fraction of retained austenite was measured by EBSP. The analysis of EBSP is aimed at the 1/4 position from the surface in the thickness direction of the hot-rolled steel plate (the area from the 1/8 depth to the 3/8 depth from the surface in the thickness direction of the surface) and the distance from the surface in the thickness direction. 1/2 position in the plate thickness direction (3/8 depth from the surface in the plate thickness direction to 5/8 depth from the surface in the plate thickness direction). The following sample is used as the sample: after grinding with #600 to #1000 silicon carbide paper, use a liquid that disperses diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water to polish to a mirror surface to fully remove The cross-section was polished by electrolytic polishing for the purpose of measuring the strain. The electrolytic polishing was performed using a mixed liquid of perchloric acid ethanol at a liquid temperature of -80°C. Here, the voltage in the electrolytic polishing may be adjusted so that the polished layer thickness of the surface layer is constant and defects due to polishing such as pits do not occur.

EBSP is measured at an accelerating voltage of 15 to 25 kV, and is measured at intervals of at least 0.25 μm or less to obtain crystal orientation information at each measurement point in the range of 150 μm or more in the thickness direction and 250 μm or more in the rolling direction. Among the obtained crystal structures, the structure whose crystal structure is fcc was judged as retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSP analyzer. The area ratio of retained austenite is obtained by obtaining the ratio of measurement points determined to be retained austenite. The obtained area ratio of retained austenite is regarded as the volume ratio of retained austenite.

Here, since a larger number of measurement points is preferable, the narrower the measurement interval and the wider the measurement range are, the better. However, when the measurement interval is smaller than 0.01 μm, adjacent points interfere with the spreading width of the electron beam. Therefore, the measurement interval is 0.01 μm or more. In addition, the measurement range can be up to 200 μm in the thickness direction and 400 μm in the rolling direction. In addition, an apparatus including a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 detector manufactured by TSL) was used for the measurement. At this time, the degree of vacuum in the apparatus was 9.6×10 ⁻⁵ Pa or less, the irradiation current level was 13, and the electron beam irradiation level was 62.

Method for Determination of Volume Ratio of Ferrite

The volume ratio of ferrite is the area ratio of crystal grains in which iron-based carbides have not been formed, obtained by observing the structure of the metallographic structure photograph. In addition, ferrite is characterized in that there is no interface generated by subgrain boundary or phase transformation in the crystal grain, and the above-mentioned iron-based carbide does not exist, and the crystal grain in which there is no interface in the crystal grain is defined as For ferrite grains. The sample is collected in such a way that the plate thickness section perpendicular to the rolling direction of the hot-rolled steel plate can be observed, and the ferrite appears by corroding the section for 10 to 15 seconds using alcohol and nital etching solution adjusted to a concentration of 3 to 5%. . Use a magnification of 500 to 1000 times to measure the 1/4 position from the surface in the thickness direction of the hot-rolled steel plate (1/8 depth from the surface in the thickness direction to 3/8 depth from the surface in the thickness direction) area) and the metallographic structure photo taken at 1/2 position from the surface in the thickness direction (3/8 depth from the surface in the thickness direction to 5/8 depth from the surface in the thickness direction) for organizational observation. An optical microscope is used in taking photographs of tissues. The metallographic structure photographs are prepared at least 3 fields of view at 1/4 position from the surface in the plate thickness direction and 1/2 position from the surface in the plate thickness direction. The area ratio of ferrite grains observed in each metallographic structure photograph was obtained, and their average value was calculated to obtain the average value of the area ratio of ferrite. This average value is regarded as the volume ratio of ferrite.

In addition, iron-based carbides were found to be black granular light and dark with an equivalent circle diameter of 1 μm or less, and were observed within crystal grains.

How to measure the volume fraction of martensite

The volume ratio of martensite is the area ratio of martensite identified from the metallographic structure photograph. Samples are collected in such a way that the thick section perpendicular to the rolling direction of the hot-rolled steel sheet can be observed, and the 1/4 position from the surface of the hot-rolled steel sheet in the thickness direction (from the surface 1/8 depth to 3/8 depth from the surface in the thickness direction of the plate) and 1/2 position from the surface in the thickness direction (3/8 depth to 3/8 depth to the surface in the thickness direction from the surface) The metallographic structure photo obtained by taking the surface in the plate thickness direction is 5/8 depth area) for structure observation. The metallographic structure is developed by etching at a liquid temperature of 60-80° C. for 30-60 seconds using a Lepera etching solution mixed with picric acid, sodium disulfite, and ethanol. In the photographed microstructure, the massive microstructure observed in shades of white is a mixed microstructure of martensite and retained austenite. The area ratio of the mixed structure of martensite and retained austenite was defined as the total volume ratio of martensite and retained austenite. The value obtained by subtracting the volume fraction of retained austenite measured by the method described above from the total volume fraction of martensite and retained austenite was defined as the volume fraction of martensite.

Determination method of volume fraction of pearlite

The volume ratio of pearlite is the area ratio of a layered structure obtained by observing the structure of the metallographic structure photograph. As the photo of the metallographic structure, the same photo used for measuring the volume ratio of ferrite can be used. The area ratio of pearlite identified from the metallographic structure photograph was defined as the volume ratio of pearlite.

Determination method of volume fraction of bainite

The volume fraction of bainite is a value obtained by subtracting the sum of the volume fractions of retained austenite, ferrite, martensite, and pearlite measured by the above-mentioned method from 100%.

Average particle size of prior austenite grains: 25.0 μm or less

As described above, the depth of the concave portion of the forming damaged portion is caused by the development of the plastically raised unevenness on the surface of the steel sheet and the contact with the die when subjected to bending or bending recovery deformation. Among them, the degree of plastic swelling on the surface of the steel sheet depends on the size of the effective grain size of the surface layer of the steel sheet. In the composition of the metallographic structure, the effective crystal grain size corresponds to the average grain size of prior-austenite grains, and is the unit of maximum deformation of prior-austenite grain boundaries. When the average grain size of prior-austenite grains exceeds 25.0 μm, the depth of the forming damage portion becomes deeper, and the fatigue properties of the forming damage portion of the hot-rolled steel sheet deteriorate. Therefore, the average grain size of the prior-austenite grains in the surface region (the region from the surface to a position 100 μm away from the surface in the plate thickness direction) is 25.0 μm or less. It is preferably 20.0 μm or less or 15.0 μm or less.

In addition, the smaller the average grain size of the prior-austenite grains, the better, but an extremely high rolling load is required to make it smaller than 3.0 μm, so it may be 3.0 μm or more.

Method for determination of average grain size of prior austenite grains

In order to measure the average particle size of prior austenite grains, samples were collected in such a way that the thickness section perpendicular to the rolling direction of the hot-rolled steel sheet could be observed, and the samples were collected using a saturated aqueous solution of picric acid and dodecylbenzenesulfonic acid. A sample in which the structure of the thick section of the plate is visualized by a sodium etching solution. The equivalent circle of prior austenite grains was measured using a photograph of the structure taken with a scanning electron microscope at a magnification of 500 times in the surface layer region of the sample (the region from the surface to a position 100 μm away from the surface in the plate thickness direction) diameter. In addition, the scanning electron microscope is equipped with 2 electron detectors. The photo of the tissue is taken in a vacuum below 9.6×10 ^-5 Pa, the sample is irradiated with an electron beam at an accelerating voltage of 15kV and an irradiation current level of 13, and the surface layer area (from the surface to the position 100 μm away from the surface in the direction of plate thickness) is photographed. area) of the secondary electron image. The number of shooting fields of view is more than 10 fields of view. In the captured secondary electron image, prior-austenite grain boundaries are photographed as bright shades. For one of the prior-austenite grains included in the observation field of view, the equivalent circle diameter is calculated. Excluding the edge of the imaging field of view and the entire prior austenite grains that are not included in the imaging field of view, perform the above operation on all prior austenite grains included in the observation field of view, and obtain all the prior austenite grains in the imaging field of view. The equivalent circle diameter of the original austenite grains. The average grain size of prior-austenite grains is obtained by calculating the average value of circle-equivalent diameters of prior-austenite grains obtained in each imaging field of view.

The maximum depth of the region where the rotation angle between the normal of the one of the faces and the (011) pole near the normal of the one of the faces is 5° or less, and the maximum depth in the other face The ratio of the normal line of the other said surface to the maximum depth of the region where the rotation angle of the (011) pole near the said normal line of the other said surface is 5° or less: 1.00˜1.20

The concave portion of the forming damage portion is formed by the development of unevenness due to the plastic deformation of the surface layer of the steel sheet during forming and contact with the die. From this, the inventors found that the depth of the recess depends on the unit of deformation of the surface layer of the steel sheet, and can be reduced in accordance with the grain size of prior-austenite in high-strength steel. However, only by controlling the grain size of prior austenite, desired fatigue properties cannot be obtained at the forming damage portion. Since high stress occurs at the most rigid portion, the fatigue damage of the component develops most in the vertical wall and the flat portion of the R portion. The R portion receives bending or bending recovery deformation such as cap molding. The inventors conducted innovative research and found that the maximum depth of the region where the normal line of the surface and the rotation angle of the (011) pole near the normal line of the surface is 5° or less is different on the front and back sides of the steel plate. The ratio of the maximum depths of the regions between the front and back surfaces determines the depth of the concave portion, and by setting this ratio to 1.00 to 1.20, desired fatigue characteristics can be obtained also in the molded damaged portion. Therefore, the maximum depth of the region where the rotation angle between the normal of one surface of the steel plate and the (011) pole in the vicinity of the normal of the one of the surfaces is 5° or less, and the other in the other surface The ratio of the normal of the surface to the maximum depth of a region where the rotation angle of the (011) pole near the normal of the other surface is 5° or less (hereinafter, sometimes simply referred to as the ratio of the maximum depth ) is 1.00 to 1.20. The maximum depth ratio is preferably 1.15 or less or 1.10 or less.

Hereinafter, a method of measuring the maximum depth of a region having a predetermined rotation angle between a normal line of one surface and a (011) pole near the normal line of the one surface will be described.

The measurement was performed by EBSP using a sample whose cross-section was mirror-polished in the same manner as the sample for measuring the volume ratio of the prior austenite grains. The sample was polished by electrolytic polishing for the purpose of sufficiently removing the strain on the measurement cross-section.

In the measurement of EBSP, the acceleration voltage can be set to 15 to 25 kV, the measurement range can be set to the total plate thickness, and the range of 1000 μm or more in the rolling direction can be set to the measurement range. In addition, since the purpose is to measure the average characteristics of the crystal orientation, the measurement interval may be 5 μm or more. In addition, in order to avoid the increase of unmeasured crystal grains, the measurement interval was set to 25 μm or less. In addition, the crystallographic orientation data were recorded in conjunction with the measured coordinate system. Based on the obtained crystal orientation data, the rotation angle between the normal to one surface of the steel sheet and the (011) pole near the normal was measured by the following method.

The rotation angle between the normal line of one surface (surface or back surface) of a hot-rolled steel sheet and the (011) pole near the normal line is a value obtained by plotting and measuring crystal orientation data obtained by EBSP measurement on a positive pole diagram. . When plotting the crystallographic orientation on the positive pole diagram, the coordinate system of the positive pole diagram has the normal line (origin ND) as the normal line to the surface of the hot-rolled steel sheet, the horizontal axis TD as the plate width direction, and the horizontal axis perpendicular to Axis RD is the form of the rolling direction and represents the pole of the (011) orientation. The above-mentioned crystal orientation is a group of points obtained by measuring 1000 μm or more in the rolling direction at predetermined intervals, and the measurement range is the range of the total plate thickness. This point group is divided into 20 parts in the plate thickness direction, and the (011) pole diagram is drawn. In the (011) pole diagram drawn in this way from one surface of the hot-rolled steel sheet at each position in the depth direction, the distance between the origin ND (the normal line of one surface of the hot-rolled steel sheet) and the closest (011) pole is measured. angle. The measured value is defined as the rotation angle between the normal of one surface and the (011) pole near the normal. At each position in the depth direction, the maximum depth of the region where the rotation angle is 5° or less is obtained.

By performing the above operations on the surface and the back of the hot-rolled steel sheet, a region having a predetermined rotation angle between the normal to the surface and the (011) pole in the vicinity of the normal to the surface is obtained on both surfaces of the hot-rolled steel sheet maximum depth. By calculating the value obtained by dividing the value of the face with the greater maximum depth among the face faces by the value of the other face, the normal of the one of the faces and the normal of the one of the faces are obtained The maximum depth of the region where the rotation angle of the (011) pole near the normal is 5° or less, and the normal of the other said surface in the other surface and the said other said surface The rotation angle of the (011) pole near the normal is the ratio of the maximum depth of the region below 5°.

Tensile strength: 1150MPa

The hot-rolled steel sheet of the present embodiment has a tensile strength of 1150 MPa or more. If the tensile strength is less than 1150 MPa, applicable automobile chassis parts are limited. Originally, when the tensile strength is less than 1150 MPa, the improvement of the fatigue properties of the molded damaged portion does not become a problem. The tensile strength may be 1200 MPa or more or 1300 MPa or more. The higher the tensile strength, the better, but it may be 1500 MPa or less from the viewpoint of the effect of reducing the weight of parts by increasing the strength of the hot-rolled steel sheet.

Using the No. 5 test piece of JIS Z 2241: 2011, a tensile test was performed in accordance with JIS Z 2241: 2011 to measure the tensile strength. The collection position of the tensile test piece is the central position in the width direction of the plate, and the direction perpendicular to the rolling direction is the length direction.

Total elongation: more than 13%

The total elongation needs to be 13.0% or more in order not to cause necking or breakage during the molding of the flange portion and the protruding portion of the undercarriage of the automobile. Therefore, the total elongation may be 13.0% or more. Preferably it is 14.0% or more.

In addition, the total elongation refers to the elongation at break in a tensile test for measuring the above-mentioned tensile strength.

Hole expansion rate: more than 35.0%

The hole expansion rate of the hot-rolled steel sheet of this embodiment may be 35.0% or more. When the hole expansion rate is less than 35.0%, molding fractures occur at the end of the cylindrical flange, and thus may not be applicable to automobile running parts. In order to further increase the molding height of the cylindrical burring portion, the hole expansion rate may be 50.0% or more.

The hole expansion test is carried out according to JIS Z 2256:2010 to measure the hole expansion rate.

Fatigue strength of molded damaged part: 350MPa or more

In the currently used 780 MPa class steel plate, the fatigue strength of the concave portion of the forming damage is not a problem, and the fatigue limit ratio is 0.45 or more. In the hot-rolled steel sheet of this embodiment, even if there is a concave portion of the forming damage, it is necessary to obtain fatigue strength equivalent to that of a steel sheet of 780 MPa class, so the fatigue strength of the forming damage is preferably 350 MPa or more. When the fatigue strength of the molded damaged part is 350 MPa or more, it can be considered that the fatigue strength of the molded damaged part is excellent.

In addition, the fatigue limit ratio refers to a value obtained by dividing the fatigue strength by the tensile strength (fatigue strength/tensile strength).

Fatigue characteristics of the concave portion of the formed damaged portion were evaluated by hat forming a short hot-rolled steel sheet and measuring the fatigue strength using a plane bending fatigue test piece produced from the formed hot-rolled steel sheet. When the vertical wall is formed in the cap molding, since it is in contact with the punch while undergoing bending recovery deformation, the concave portion of the flat -R portion formed near the vertical wall portion of the running member can be reproduced. The short strip-shaped hot-rolled steel sheet used for cap molding has a dimension of 35 mm in width and 400 mm in length, with the longitudinal direction being the L direction. This short hot-rolled steel sheet was cap-formed using a square-headed punch of about R6. In the molding test, model 145-100 manufactured by ERICHSEN was used. A test piece having a shape conforming to JIS Z2275:1978 was produced from the vertical wall of the molded cap test piece, and a fatigue test was performed. Fatigue test conditions were set at room temperature as stress ratio R=-1, frequency 25 Hz, load was repeatedly applied up to 10 ⁶ times, and the number of repetitions of fracture was measured. The stress without breaking up to 10 ⁶ times was taken as the fatigue strength.

The hot-rolled steel sheet of the present embodiment having the chemical composition and metallographic structure described above may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., and may be used as a surface-treated steel sheet. The coating can be electroplated or hot dipped. As the plated layer, galvanized plated, Zn—Ni alloy plated, etc. can be exemplified. As the hot-dip coating, hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip aluminum, hot-dip Zn-Al alloy, hot-dip Zn-Al-Mg alloy, hot-dip Zn-Al- Mg-Si alloy, etc. The plating deposition amount is not particularly limited, and may be the same as conventional ones. In addition, the corrosion resistance can be further improved by performing an appropriate chemical conversion treatment (for example, coating and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

Next, a preferred method of manufacturing the hot-rolled steel sheet of the present embodiment will be described. The casting process and the hot rolling process described below are requirements for reducing the depth of the concave portion of the molded damaged portion, and are important processes for controlling the crystal orientation in the plate thickness direction and the average grain size of prior-austenite grains.

A preferred method of manufacturing the hot-rolled steel sheet of this embodiment includes the following steps.

In the continuous casting process, when continuously casting a slab having a predetermined chemical composition, the average cooling rate gradient of the meniscus to the surface temperature in the region 1.0 m away from the meniscus is 0.20 to 15.00°C/ s ² to obtain the slab by continuous casting;

heating step, heating the slab to above 1200°C;

A hot rolling step, after rough rolling the heated slab, rolling in the temperature range of 870 to 980°C at a total reduction rate of 80% or more in the temperature range of 870°C to 980°C The finish rolling is carried out in such a way that the elapsed time between the rolling stands is 4.00 seconds or less;

cooling process, cooling to a temperature range of 300-550°C; and

In the coiling step, coiling is performed so that the coiling temperature becomes the above-mentioned temperature range of 300 to 550°C.

Hereinafter, each step will be described.

continuous casting process

When continuously casting a slab having the above-mentioned chemical composition, the average cooling rate gradient of the surface temperature in the region from the meniscus to 1.0 m from the meniscus is 0.20 to 15.00° C./s ² . In addition, in the present embodiment, the average cooling rate gradient of the surface temperature refers to the time change of the cooling rate in the range of 1.0 from the meniscus. The average cooling rate gradient can be calculated based on temperature data obtained from thermometers embedded in the mold copper plate at distances of 0.1 m, 0.5 m, and 1.0 m from the meniscus. Let the temperature measurement values at a position at a distance of 0.1 m, 0.5 m, and 1.0 m from the meniscus at a certain time be T _0.1 , T _0.5 , and T _1.0 . In the case where the solidified shell is at a position 0.1m away from the meniscus as t _0.1 , if the casting speed is set as V (m/sec), then the solidified shell passes through the position 0.5m away from the meniscus as t _0.5 = (t _0.1 +0.4/V). Similarly, the above-mentioned solidified shell passes through the position 1.0 m away from the meniscus as t _1.0 =(t _0.1 +0.9/V). If t _0.1 , t _0.5 , t _1.0 with the above relationship and the temperature measurement values T _0.1 , T _0.5 and T _1.0 at each position are used to represent the cooling rate gradient within 1.0m from the meniscus, it is (4/9 )×V ² ×T _1.0 +(5/9)×V ² ×T _0.1 −(1.62/1.25)×V ² ×T _0.5 .

For an arbitrary certain time from the start to the end of the continuous casting of the target steel, the cooling rate gradient was obtained for the front and back surfaces of the slab, and the average value thereof was taken as the cooling rate gradient at that time. At least 20 points or more of the cooling rate gradient at this certain time were measured, and the average value thereof was defined as the average cooling rate gradient of the surface temperature in the region from the meniscus to 1.0 m from the meniscus. In addition, a maximum of 100 points can be measured.

The cooling rate of the surface temperature affects the growth of columnar crystals in the early stage of solidification, and its gradient affects the generation frequency of columnar crystal clusters in the surface layer. When the average cooling rate gradient of the surface temperature in the region from the meniscus to 1.0 m from the meniscus exceeds 15.00° C./s ² , the maximum depth ratio exceeds 1.20. The smaller the average cooling rate gradient in the above range is, the more preferable it is, but since cooling control is extremely difficult when it is less than 0.20°C/s ² , it is preferably 0.20°C/s ² or more.

The average casting speed in the continuous casting process may be in a general range, and may be 0.8 m/min or more, or may be 1.2 m/min or more. From the viewpoint of cost reduction, it is preferably 1.2 m/min or more. On the other hand, when the average casting speed exceeds 2.5 m/min, defects in the slab during solidification tend to occur. Therefore, the average casting speed is preferably 2.5 m/min or less.

In addition, when the average casting speed is less than 0.6 m/min, the cooling temperature gradient in the thickness direction of the slab decreases, but the economical efficiency is significantly impaired. Therefore, the average casting speed is preferably 0.6 to 2.5 m/min. In addition, the cooling temperature gradient mentioned here is different from the above-mentioned average cooling rate gradient.

heating process

The slab obtained by continuous casting is heated to a surface temperature of 1200° C. or higher for solid solution. When the slab contains Ti, the heating temperature is preferably 1230° C. or higher in order to more reliably dissolve Ti. In addition, the slab temperature before heating may be a slab cooled to room temperature, and if there is concern about cracks due to thermal stress or the like, the high temperature after continuous casting may be maintained. Heating in the heating step is carried out by placing the slab in a furnace controlled to a predetermined temperature, but it is sufficient to keep the slab surface temperature at 1200° C. or higher (holding time) for 30 minutes or more. In addition, when the slab contains Ti, it is sufficient to set the heating temperature to 1230° C. or higher for a time (holding time) of 30 minutes or longer. The upper limit of the holding time may be 300 minutes or less. In the furnace, a slab is arranged on a mat of an inorganic substance, and at this time, it is only necessary to heat and form a solid solution at a temperature below a temperature at which the heated slab does not melt due to the reaction between the inorganic substance and iron. For example, the heating temperature may be 1400°C or lower.

hot rolling process

After heating the slab, rough rolling is performed, and then finish rolling is performed within the range described below. The finish rolling was performed so that the total rolling reduction in the temperature range of 870-980 degreeC was 80 % or more. When the finish rolling temperature exceeds 980°C, the average grain size of austenite grains becomes large irrespective of the total rolling reduction in the rolling stand, the depth of the forming damage cannot be reduced, and excellent rolling cannot be obtained at the forming damage. Fatigue properties.

When the total reduction rate in the temperature range of 870 to 980° C. is less than 80%, the average grain size of prior austenite grains exceeds 25.0 μm. The total rolling reduction mentioned here means the value obtained by adding the rolling reductions of each rolling stand whose nip temperature is 870-980 degreeC. The upper limit of the total reduction ratio in the temperature range of 870 to 980° C. may be 95% or less.

In addition, in the hot rolling process, when the ratio of the rough-rolled plate thickness t ₀ to the finish-rolled product plate thickness t, that is, the total plate reduction rate ((1-t/t ₀ )×100) is less than 80%, regardless of No matter how the rolling temperature is controlled, the total reduction rate in the temperature range of 870 to 980° C. cannot be made to be 80% or more, so the total reduction rate is limited to 80% or more. The higher the total sheet reduction rate, the higher the yield, which is preferable. However, if it exceeds 98%, the load on the rolling mill will increase, and the cost of roll replacement and the like will increase. In consideration of the load on the rolls, the total sheet loss is more preferably 95% or less. Therefore, the ratio of the rough-rolled sheet thickness to the finish-rolled product sheet thickness, that is, the total sheet reduction rate is limited to 80% or more. In addition, the total sheet loss is preferably 98% or less.

The total number of rolling stands is not particularly limited, and may be determined according to the capacity of the rolling mill such as load resistance or torque. Generally, the number of rolling stands whose bite temperature is 870-980 degreeC is 2 or more stands. In finish rolling in the temperature range of 870 to 980°C, when the elapsed time between rolling stands exceeds 4.00 seconds, austenite grains grow in this range, and the average size of prior austenite grains is Since the diameter exceeds 25.0 μm, it is not preferable. Therefore, when the number of rolling stands whose nip temperature is 870-980 degreeC exceeds 2, the elapsed time between rolling stands is 4.00 second or less. When the elapsed time between rolling stands is less than 0.30 seconds, since the load on the rolling rolls increases, the above-mentioned elapsed time may be 0.30 seconds or more.

In addition, what is necessary is just to obtain|require a temperature of the steel plate surface measured with the thermometer, such as the radiation thermometer installed in each frame, about a bite temperature.

cooling process

After finish rolling, it is cooled to a temperature range of 350 to 550°C. If the cooling stop temperature after finish rolling is outside the temperature range of 330 to 550° C., the coiling described later cannot be performed within the desired temperature range.

Coiling process

After cooling, in order to make the intensity|strength of a hot-rolled steel sheet into 1150 MPa or more, it coils so that a coiling temperature may become the temperature range of 350-550 degreeC. When the coiling temperature is lower than 350° C., the volume ratio of martensite increases. Therefore, the coiling temperature is 350° C. or higher. Preferably it is 380°C or higher. On the other hand, when the coiling temperature exceeds 550°C, the volume fraction of bainite decreases, and furthermore, when the temperature exceeds 570°C, the volume fraction of ferrite increases. Therefore, the crimping temperature is 550° C. or lower. Preferably it is 480°C or lower.

The coiling temperature may be the average value of the surface temperature of the steel sheet over the entire length of the coil measured over the entire length of the coil using a thermometer installed in the section from the cooling device to the coiler after cooling. This is because the average value of the surface temperature of the steel sheet over the entire length of the coil is equivalent to the temperature of the coil after being wound into a coil. In addition, in order to reduce material variation in the steel coil, the coiling temperature at any point of the steel coil is preferably 480° C. or lower at the maximum. That is, the steel sheet surface temperature is preferably 480° C. or lower over the entire length of the steel coil.

In the present embodiment, it is preferable that the elapsed time from the start of cooling in the cooling step to the start of coiling in the coiling step be 30 seconds or less. The elapsed time here refers to the time from the completion of finish rolling to the start of coiling. In the chemical composition of the hot-rolled steel sheet of this embodiment, the cooling time is not particularly limited, but in the case of a long cooling time, since the length of the air-cooled zone in the cooling zone becomes longer, the thickness of the oxide scale on the surface layer becomes thicker, so the acid The cost in the washing process increases. Therefore, the cooling time is preferably 30 seconds or less. In addition, in the cooling process, the adjustment of the elapsed time from the start of cooling to the start of coiling can be performed by adjusting the average cooling rate of the cooling in the cooling process. The cooling method after finish rolling is water cooling or air cooling on the run-out table, and the cooling method may be selected so as to obtain a desired cooling time.

The hot-rolled steel sheet produced by the above method may be left to cool to room temperature, or may be water-cooled after being coiled. After allowing to cool to room temperature, it may be uncoiled and pickled again, or skin pass rolling for residual stress and shape adjustment may be performed.

Tempering process

A preferable manufacturing method of the hot-rolled steel sheet according to the present embodiment may further include a tempering step of tempering the hot-rolled steel sheet manufactured through the above-mentioned steps in order to further improve ductility.

In the case of tempering, it is preferably carried out under the condition of maintaining the temperature for 90 to 80,000 seconds in the temperature range of 200°C to less than 450°C. When the tempering temperature is lower than 200° C., the change in the material cannot be confirmed almost, and the production cost increases due to an increase in the number of steps, which is not preferable. In addition, when the tempering temperature is 450°C or higher, the pearlite fraction exceeds 3.0%, and the hole expandability decreases. The average temperature increase rate in the tempering step is not particularly limited, but may be 0.01° C./second or more in order not to lower the heat treatment efficiency. In addition, the atmosphere during tempering may be an oxidizing atmosphere or an atmosphere substituted with N or the like. Tempering can also be performed on a coil-shaped hot-rolled steel sheet, but in this case, in order to reduce variation in the steel coil, the holding time is preferably 1000 seconds or more.

After the tempered hot-rolled steel sheet is cooled to room temperature, pickling for removing scale formed during hot rolling or heat treatment may be performed as needed.

Plating process

A preferred manufacturing method of the hot-rolled steel sheet according to this embodiment may further include a plating step of subjecting the hot-rolled steel sheet manufactured by the above method or the hot-rolled steel sheet after the tempering step to a hot-dip galvanizing treatment.

When hot-dip galvanizing is performed, it is preferable to set the maximum temperature to a temperature range of 450 to 495° C., and to set the residence time in this temperature range to 75 seconds or less. The residence time at less than 450°C is the same as the above-mentioned tempering step, and may be a residence time of 90 to 80,000 seconds in the temperature range of 200°C or higher and less than 450°C. If the maximum temperature exceeds 495° C., the volume fraction of retained austenite will be less than 3.0% regardless of the residence time, and the ductility of the hot-rolled steel sheet after plating will decrease. If the maximum temperature is less than 450° C., defects will be generated in the plating layer, which is not preferable. As for other conditions, the method of applying plating is not particularly limited as long as it is within the range of the above-mentioned temperature history. The plating deposition amount is not particularly limited, and may be the same as conventional ones. In addition, the corrosion resistance can be further improved by performing an appropriate chemical conversion treatment (for example, coating and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

[Example]

Slabs having the chemical compositions shown in Table 1 were produced by continuous casting. The conditions of continuous casting are the conditions described in Table 2-1 and Table 2-2. In the continuous casting, in Test Nos. 4, 5, 10, 13 and 19, the average cooling rate gradient of the surface temperature in the region from the meniscus to 1.0 m from the meniscus exceeded 15.00° C./sec ² .

According to the conditions shown in Table 2-1 and Table 2-2, hot-rolled steel sheets having a thickness of 2.6 mm were produced from the obtained slabs. Tempering and plating were performed under the conditions shown in Table 2-1 and Table 2-2 as needed. In addition, in cooling after hot rolling, it cooled to the coiling temperature described in Table 2-1 and Table 2-2. In addition, the elapsed time from the start of cooling in the cooling step to the start of coiling in the coiling step is 30 seconds or less.

[Table 1]

[table 2-1]

[Table 2-2]

[Table 3-1]

[Table 3-2]

In Test Nos. 17 to 20, 23 and 25, after hot rolling, the steel coil was uncoiled, and the steel plate was cut so as to have a size capable of performing predetermined property evaluation, and heat treatment (tempering) in a box furnace was performed. In addition, in test numbers 27 and 28, a hot-dip galvanized coating was provided by performing a plating treatment under the conditions shown in Table 2-2.

Using the No. 5 test piece of JIS Z 2241: 2011, the tensile test was carried out in accordance with JIS Z 2241: 2011. The tensile strength was obtained from the point showing the maximum load, and the total elongation was obtained from the displacement at break. The collection position of the tensile test piece is the central position in the width direction of the plate, and the direction perpendicular to the rolling direction is the length direction.

When the tensile strength is 1150 MPa or more, it is considered to have excellent strength and judged to be acceptable, and when the tensile strength is less than 1150 MPa, it is considered to not have excellent strength and judged to be unacceptable.

The hole expansion test is carried out according to JIS Z 2256:2010 to measure the hole expansion rate. When the total elongation is 13.0% or more and the hole expansion rate is 35.0% or more, it is considered to have excellent formability and judged to be acceptable. On the other hand, as long as any one is not satisfied, it is considered that it does not have excellent moldability, and it is judged as unacceptable.

The fatigue properties of the formed damaged portion were evaluated based on the fatigue strength obtained by subjecting the obtained hot-rolled steel sheet to cap forming and performing a fatigue test on the formed hot-rolled steel sheet. The conditions of the fatigue test are as described above.

When the fatigue strength is 350 MPa or more, the fatigue properties of the molded damaged part are considered to be excellent and judged to be acceptable.

In Test No. 29 with a low C content, the amount of retained austenite was small, and the total elongation was less than 13.0%.

In Test No. 30 with high C content and Si content, and Test No. 31 with high Si content, the volume ratio of retained austenite was high, and the hole expansion rate was low.

In Test No. 32 with low Mn content and Test No. 36 with low B content, the tensile strength was less than 1150 MPa.

In Test No. 37 with a low Si content, the volume ratio of retained austenite was low, and the total elongation was low. In Test No. 38 in which the Nb content was low, the prior-austenite grains were coarse, and the fatigue strength of the forming damage portion was low.

In addition, in Test Nos. 33 to 35, hot rolling could not be performed due to nozzle clogging during casting and microcracks at the corners, and thus hot rolled steel sheets could not be produced.

Even if the chemical composition is within the scope of the present invention, the total rolling reduction in the temperature range of 870-980°C is less than 80% in Test No. 9, and the rolling stand between the rolling stands in the temperature range of 870-980°C In Test No. 12 in which the maximum elapsed time exceeded 4.00 seconds, the prior-austenite grains were coarse, and the fatigue strength of the forming damage portion was low.

Test No. 7 in which the coiling temperature was low had a low hole expansion ratio.

Test No. 16 having a high coiling temperature had a low volume fraction of bainite and a low hole expansion rate.

Test No. 15 having a high coiling temperature had a high volume ratio of ferrite, a tensile strength of less than 1150 MPa, and a low hole expansion rate.

Among Test Nos. 17 to 20, 23, and 25 that were tempered after hot rolling, Test No. 18 in which the tempering temperature exceeded 450° C. had a high pearlite volume ratio and decreased hole expansion ratio.

In Test No. 27 and Test No. 28 among 28 in which the plating treatment was performed, since the highest temperature exceeded 495° C., the volume ratio of pearlite increased and the hole expansion rate decreased.

The fatigue strength of the forming damaged part is different from the structural factors governing the tensile strength, total elongation and hole expansion rate. Ratio (the normal of said one of said surfaces in one surface and the rotation angle of the (011) pole near said normal of said one of said surfaces is the maximum depth of the region of 5° or less, and the other surface The ratio of the normal of the other said surface and the maximum depth of the region where the rotation angle of the (011) pole near the normal of the other said surface is 5° or less) dominates. In addition, as shown in FIG. 3 , it can be seen that the ratio of the maximum depth is particularly dominated by the average cooling rate gradient of the surface temperature in the region from the meniscus to 1.0 m from the meniscus. As shown in FIG. 3 , in the range of 0.20° C. to 15.00° C./sec ² , the maximum depth ratio is 1.20 or less, and the fatigue strength of the molded damaged portion is 350 MPa or more.

industrial availability

According to the above aspect of the present invention, there can be provided a hot-rolled steel sheet having high strength, excellent formability, and excellent fatigue properties at a formed damaged portion, and a method for producing the same. According to the above aspect of the present invention, since the fatigue characteristics of the formed damaged portion are excellent, it is possible to provide a hot-rolled steel sheet capable of reducing the depth of the concave portion in the flat portion near the R portion formed when the R portion is formed, and its manufacturing method. .

Claims (5)

1. A hot rolled steel sheet, characterized in that,

the chemical component comprises the following components in mass percent

C：0.085～0.190％、

Si：0.40～1.40％、

Mn：1.70～2.75％、

Al：0.01～0.55％、

Nb：0.006～0.050％、

P: less than 0.080 percent,

S: less than 0.010 percent,

N: less than 0.0050%,

Ti：0.004～0.180％、

B：0.0004～0.0030％、

Mo：0～0.150％、

V：0～0.300％、

Cr:0 to 0.500 percent

Ca：0～0.0020％，

The rest part is composed of Fe and impurities;

1/4 of the position in the plate thickness direction from the surface and 1/2 of the position in the plate thickness direction from the surface, in volume percent

The residual austenite is 3.0-12.0%,

More than 75.0 percent of bainite and less than 97.0 percent,

Ferrite of 10.0% or less,

Martensite is 10.0% or less,

Pearlite is 3.0% or less;

in a metallographic structure of the surface to a region at a position of 100 [ mu ] m from the surface in a plate thickness direction, an average grain size of prior austenite grains is 25.0 [ mu ] m or less;

A ratio of a maximum depth of a region in which a rotation angle of a normal line of one of the faces to a (011) pole in the vicinity of the normal line of the one face is 5 ° or less to a maximum depth of a region in which a rotation angle of a normal line of the other face to a (011) pole in the vicinity of the normal line of the other face is 5 ° or less is 1.00 to 1.20;

the tensile strength is 1150MPa or more.

2. The hot rolled steel sheet according to claim 1,

the chemical component contains one or more than two of the group consisting of the following elements in mass%:

Mo：0.030～0.150％、

V：0.050～0.300％、

cr:0.050 to 0.500%

Ca：0.0006～0.0020％。

3. A method for producing a hot-rolled steel sheet according to claim 1 or 2, comprising:

a continuous casting step of continuously casting a slab having the chemical composition according to claim 1, wherein an average cooling rate gradient of a meniscus to a surface temperature in a region 1.0m from the meniscus is 0.20 to 15.00 ℃/s ² Continuously casting to obtain the slab;

a heating step of heating the slab to 1200 ℃ or higher;

a hot rolling step of rough rolling the heated slab, and then finish rolling the slab so that a total rolling reduction in a temperature range of 870 to 980 ℃ is 80% or more and an elapsed time between rolling stands in the temperature range of 870 to 980 ℃ is 4.00 seconds or less;

A cooling step of cooling to a temperature range of 300-550 ℃; and

and a winding step of winding the steel sheet at a winding temperature within the above-mentioned temperature range of 300-550 ℃.

4. A method for producing a hot rolled steel sheet according to claim 3,

the method comprises a tempering step, wherein the tempering step is performed after the coiling step, and the tempering step is performed for 90-80000 seconds in a temperature range of more than 200 ℃ and less than 450 ℃.

5. A method for producing a hot rolled steel sheet according to claim 3 or 4,

the method comprises a plating step of hot-dip galvanization of a hot-rolled steel sheet after the coiling step or a hot-rolled steel sheet after the tempering step by a thermal history having a residence time of 75 seconds or less in a temperature range of 450 to 495 ℃.

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